Abstract: Brain development is a highly dynamic yet precisely orchestrated process. Using genetically modified mouse models, we are in the process of unveiling the complex mechanisms that control critical cellular events in the developing brain. High-throughput imaging tools will greatly benefit studies in this area by charactering brain phenotypes at the macroscopic/mesoscopic levels and directing subsequent examinations at the cellular and molecular levels. In this project, multiple novel magnetic resonance imaging (MRI) techniques will be developed to non-invasively exam a wide range of phenotypes in the developing mouse brain from mid- embryonic stage to adolescence. The target phenotypes include macroscopic brain morphology and structural connectivity, microstructural organization, neuronal migration and differentiation, and postnatal brain activity. The proposed techniques include fast imaging sequences, novel image contrasts, optimized imaging coils/holder, and image analysis tools, many of which stem from on our existing expertise. In Aim 1, we will develop imaging tools to achieve high-throughput in vivo multi-contrast MRI of the developing mouse brain. We will collect multi-contrast MRI data to construct an in vivo MRI atlas of the developing mouse brain to assist mouse brain phenotype analysis and use the sas4-/- mouse, a model of microcephaly, to test the performance of the technique. In Aim 2, we will use novel diffusion MRI techniques to characterize macroscopic morphology, connectivity, and microstructural organization in the developing brain. In particular, high angular resolution diffusion imaging (HARDI) will be used to resolve complex tissue microstructural organization and reconstruct connectivity between major brain regions, and the new oscillating gradient diffusion MRI technique will be used to exam changes in cellularity in the developing cortex associated with neuronal migration. Detailed examination of the relationships between diffusion MRI-based markers and specific histological markers will determine their sensitivity to the underlying developmental processes. In Aim 3, we will use novel Manganese (Mn2+)-enhanced MRI as another tissue contrast, which reflects postnatal brain activity and potentially neuronal differentiation in the embryonic brain, to examine the developing mouse brain. We will examine the contrast patterns of Mn2+-enhanced MRI in the embryonic and neonatal mouse brain with the patterns of neuronal differentiation observed in histological data to determine the sensitivity of Mn2+-enhanced MRI to neuronal differentiation. In addition, we will investigate potential toxic effects of Mn2+ on brain development, and establish protocols that minimize these effects. In Aims 2 and 3, the techniques will also be used to characterize three mutant mouse models with abnormal brain phenotypes resulting from defects in neuronal migration and differentiation. The imaging techniques and knowledge gained in this project will greatly enhance our ability to quantitatively characterize the phenotypes of mutant mouse models in order to achieve a deep understanding of brain development and disorders.